In automotive technology “brake-by-wire” brake systems are being used increasingly widely. Such brake systems often include, in addition to a brake master cylinder which can be actuated by the vehicle driver, an electrically controllable pressure supply device by means of which actuation of the wheel brakes or of the brake master cylinder takes place in the brake-by-wire operating mode. In order to communicate an agreeable pedal feel to the vehicle driver in the brake-by-wire operating mode, the brake systems usually include a brake pedal feel simulator. In these brake systems the wheel brakes can also be actuated on the basis of electronic signals without active intervention by the vehicle driver. These electronic signals may be output, for example, by an electronic stability program or a distance control system.
A brake-by-wire brake system comprising a brake master cylinder actuable by the brake pedal, a travel simulator and a pressure supply device is known from the international patent application WO 2011/029812 A1, which is incorporated by reference. In a brake-by-wire operating mode the wheel brakes are subjected to pressure by the pressure supply device. In a fallback operating mode the wheel brakes are subjected to pressure by means of the brake master cylinder actuable by the brake pedal. For each wheel brake the brake system includes an inlet valve and an outlet valve for adjusting wheel-individual brake pressures. A method for activating these valves is not described.
WO 2010091883 A1, which is incorporated by reference, discloses a brake system having a brake booster the piston-cylinder system of which is driven mechanically by an electric motor, a single 2/2-way switching valve, a so-called multiplex valve, being provided for adjusting a wheel-individual brake pressure. With the multiplex method, pressure is adjusted sequentially at the individual wheel brakes, the pressure adjustment being effected by the forward and return stroke of the piston of the piston-cylinder system. Here, the not inconsiderable noise generation is considered disadvantageous, which noise results inter alia from the fact that, firstly, the electric motor has to execute high-frequency reversing operations of the piston-cylinder system and, secondly, the multiplex valves have to be closed and opened in rapid succession, causing impact noises inside the multiplex valves.